Asthma

Detailed Overview

The pathophysiology of asthma primarily involves type 2 (Th2-driven) inflammation, leading to eosinophilic infiltration, IgE production, and mast cell degranulation. Recurrent inflammation can result in 'airway remodeling'—permanent structural changes including basement membrane thickening and smooth muscle hypertrophy. Diagnosis hinges on demonstrating reversible airflow limitation on spirometry. Treatment has shifted significantly; current GINA guidelines no longer recommend SABA monotherapy, instead advocating for ICS-containing inhalers (like ICS-Formoterol) as the primary reliever to control underlying inflammation and prevent fatal exacerbations.

Epidemiology & Demographics

Affects approximately 339 million people globally. In children, it is more common in boys, but in adults, it is more common in women. Often begins in childhood but can manifest at any age.

Etiological Mechanism

A complex interaction between genetic predisposition (atopy) and environmental exposures. Allergic asthma is driven by allergens (dust mites, pollen, pet dander). Non-allergic triggers include viral respiratory infections, exercise, cold air, and stress.

Primary Causes

Sensitization to environmental allergens in genetically susceptible individuals triggers an IgE-mediated immune response, leading to chronic airway inflammation.

• Atopy: The strongest risk factor; personal or family history of allergic rhinitis, eczema, or food allergies.
• Occupational Exposure: Exposure to isocyanates, flour, or wood dust can induce occupational asthma.
• Obesity: Associated with more severe, poorly controlled, non-eosinophilic asthma phenotypes.
• Aspirin/NSAID use: In susceptible patients, leads to Samter's Triad (asthma, nasal polyps, aspirin sensitivity) by shunting arachidonic acid to leukotrienes.

Inhalation of a trigger causes cross-linking of IgE on mast cells, releasing histamine, leukotrienes, and prostaglandins. This causes acute bronchoconstriction. Hours later, a late-phase reaction mediated by Th2 cells, IL-4, IL-5, and IL-13 recruits eosinophils, causing mucosal edema and thick mucus plugging. Over years, chronic inflammation drives airway remodeling, leading to subepithelial fibrosis, smooth muscle hyperplasia, and fixed airflow limitation.

Characteristic Clinical Presentation

• Wheezing: High-pitched whistling sound during expiration.
• Chronic Cough: Often worse at night or early morning; sometimes the only symptom (cough-variant asthma).
• Chest Tightness: Described as a band around the chest preventing a deep breath.

Physical Examination Signs

• Prolonged expiratory phase on lung auscultation.
• Polyphonic expiratory wheezes across all lung fields.
• Use of accessory muscles (sternocleidomastoid, intercostal retractions) during severe exacerbations.

• Status Asthmaticus: Life-threatening acute severe asthma refractory to initial bronchodilator therapy.
• Airway Remodeling: Irreversible loss of lung function mimicking COPD if long-term inflammation is uncontrolled.

Diagnostic Criteria & Guidelines

Clinical history plus Spirometry: FEV1/FVC ratio < 0.70 demonstrating obstruction, WITH reversibility defined as an increase in FEV1 of > 12% and > 200 mL after administering a short-acting bronchodilator. If baseline spirometry is normal, a methacholine challenge test showing a 20% drop in FEV1 confirms bronchial hyperresponsiveness.

Differential Diagnosis

• COPD (typically irreversible, older age, smoking history)
• Vocal Cord Dysfunction (inspiratory stridor, fails to respond to bronchodilators)
• Gastroesophageal Reflux Disease (GERD - can trigger chronic cough/micro-aspiration)
• Congestive Heart Failure ('Cardiac asthma')

Laboratory Tests & Biomarkers

• Absolute Eosinophil Count: Often elevated (>300 cells/mcL) in Type 2 asthma phenotypes.
• Fractional Exhaled Nitric Oxide (FeNO): Elevated (>50 ppb), serving as a biomarker of eosinophilic airway inflammation.
• Serum IgE: Elevated in allergic asthma; used to dose anti-IgE biologic therapy.

Imaging Modalities & Findings

• Chest X-Ray:

• Intermittent
  Symptoms < 2 days/week, nighttime awakenings < 2 times/month. Normal FEV1 between exacerbations.
• Mild Persistent
  Symptoms > 2 days/week but not daily. Minor limitation in normal activity.
• Severe Persistent
  Symptoms throughout the day, frequent night awakenings, FEV1 < 60% predicted. Extreme limitation in activity.

Second-Line & Adjunctive Therapy

For moderate-to-severe asthma: Medium or high-dose ICS/LABA combinations. Add-on therapy includes long-acting muscarinic antagonists (LAMA) like Tiotropium 1.25 mcg 2 puffs daily, or Leukotriene receptor antagonists (Montelukast 10 mg PO daily).

Surgical & Procedural Management

Bronchial thermoplasty: a bronchoscopic procedure delivering radiofrequency energy to the airway walls to reduce smooth muscle mass. Reserved for severe, uncontrolled asthma despite maximal medical therapy.

Recommended Lifestyle Changes

• Identify and ruthlessly avoid triggers (e.g., use allergen-impermeable mattress covers, keep pets out of the bedroom).
• Get annual influenza and pneumococcal vaccines.
• Treat comorbidities like GERD, obesity, and allergic rhinitis.

Patient Counseling & Advice

Thoroughly review inhaler technique; poorly executed technique means medication lands in the mouth instead of the lungs. Prescribe and explain a written Asthma Action Plan (Green, Yellow, Red zones based on Peak Flow).

Follow-Up & Monitoring Schedule

Assess asthma control every 3-6 months using the Asthma Control Test (ACT). Perform spirometry yearly to assess lung function trajectory.

Preventive Strategies

Strict adherence to daily controller medication (ICS) is the primary prevention against life-threatening exacerbations and long-term lung scarring.

Generally excellent with appropriate medical management. Most patients lead entirely normal lives with unrestricted athletic capability. However, severe asthma exacerbations still cause thousands of preventable deaths annually.